Puncture-healing thermoplastic resin carbon-fiber-reinforced composites and method of fabricating the same

ABSTRACT

A composite comprising a combination of a self-healing polymer matrix and a carbon fiber reinforcement is described. In one embodiment, the matrix is a polybutadiene graft copolymer matrix, such as polybutadiene graft copolymer comprising poly(butadiene)-graft-poly(methyl acrylate-co-acrylonitrile). A method of fabricating the composite is also described, comprising the steps of manufacturing a pre-impregnated unidirectional carbon fiber preform by wetting a plurality of carbon fibers with a solution, the solution comprising a self-healing polymer and a solvent, and curing the preform. A method of repairing a structure made from the composite of the invention is described. A novel prepreg material used to manufacture the composite of the invention is described.

CROSS-REFERENCE TO RELATED APPLICATION

This patent application is a divisional of U.S. Non-Provisional patentapplication Ser. No. 14/881,899 filed on Oct. 13, 2015, which was adivisional application of U.S. Non-Provisional patent application Ser.No. 13/774,422 filed on Feb. 22, 2013 (issued as U.S. Pat. No. 9,156,957on Oct. 13, 2015), which claims the benefit of and priority to U.S.Provisional Application Ser. No. 61/602,717, filed Feb. 24, 2012, for“Puncture Healing Thermoplastic Resin Carbon Fiber Reinforced CompositesTowards More Damage/Impact Tolerant Systems”. The contents of theforegoing applications are hereby incorporated by reference in theirentireties.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT

This invention described herein was made in the performance of workunder a NASA contract and by employees of the United States Governmentand is subject to the provisions of Public Law 96-517 (35 U.S.C. § 202)and may be manufactured and used by or for the Government forgovernmental purposes without the payment of any royalties thereon ortherefor.

FIELD OF THE INVENTION

The present invention relates generally to the field of damage- andimpact-tolerant carbon-fiber-reinforced composites [“CFRCs”] and moreparticularly to damage- and impact-tolerant CFRC structures forstructural aerospace applications.

BACKGROUND OF THE INVENTION

CFRCs are known in the art. They generally comprise a matrix, such as apolymer resin, for example an epoxy or other polymer, and areinforcement of carbon fibers. The carbon fibers might also containother fibers, such as aluminum or glass. Structures of various aerospaceapplications are often made in whole or in part of CFRC.

The initiation and propagation of damage ultimately results in failureof aerospace structural components. Typical structural repairs oftenresult in damaging practices, where material is ground away and holesare drilled to secure patches, which can act as new sites for damage. Byhealing known damage or by providing healing material to areas that aresuspected to incur damage, improved safety can be realized.Damage-tolerant, self-healing structural systems provide a route towardsthis objective. Effective self-repair, however, requires that thesematerials heal quickly following low- and mid-velocity impacts, whileretaining structural integrity. Although there are materials known topossess this characteristic, such is not the case for structuralengineering systems.

Self-healing materials display the unique ability to mitigate incipientdamage and have built-in capability to substantially recover structuralload transferring ability after damage. Structures that make use ofself-healing engineering materials produce a healing response from achange in the material's chain mobility as a function of the damagemechanism/condition involved. This type of material will possess bettermechanical properties, healing capability at elevated temperatures,faster healing rates (less than 100 microseconds), and healing withoutthe need of foreign inserts or fillers (via structural chemistry). Thesematerials might have application as structural aerospace applications.

In recent years, researchers have studied different “self-healingmechanisms” in materials as a collection of irreversible thermodynamicpaths where the path sequences ultimately lead to crack closure orresealing. Crack repair in polymers using thermal and solvent processes,where a healing process triggered with heating or with a solvent hasbeen studied. A second approach involves the autonomic healing concept,where healing is accomplished by dispersing a microencapsulated healingagent and a catalytic chemical trigger within an epoxy resin to repairor bond crack faces and mitigate further crack propagation. A relatedapproach, the microvascular concept, utilizes brittle hollow glassfibers in contrast to microcapsules filled with epoxy hardener anduncured resin in alternate layers, with fluorescent dye. An approachingcrack ruptures a hollow glass fiber, releasing a healing agent into thecrack plane through capillary action. A third approach utilizes apolymer that can reversibly re-establish its broken bonds at themolecular level by either thermal activation (e.g., based on Diels-Alderrebonding), or ultraviolet light. A fourth approach, structurallydynamic polymers, are materials that produce macroscopic responses froma change in the materials' molecular architecture without heat orpressure. A fifth approach, self-healing fiber-reinforced composites,involves integrating self-healing resins into fiber reinforcedcomposites. Various chemistries have been used based on some of theaforementioned approaches described above. Although significant recovery(>90 percent) of virgin neat resin material properties have beenreported, this range has not been the case for fiber-reinforcedcomposites made from them.

The aforementioned self-healing approaches that address the repair ormitigation of crack growth and various damage conditions in materials,have the following disadvantages: (1) Slow rates of healing; (2) Use offoreign inserts in the polymer matrix that may have detrimental effectson composite performance; (3) Samples have to be held in intimatecontact or under load and/or fused together under high temperature forlong periods of time; and (4) The material may not be considered astructural load-bearing, material.

For example, ionomers containing ionic groups at low concentrations (<15mol percent) along the polymer backbone. In the presence ofoppositely-charged ions, these ionic groups form aggregates that can beactivated by external stimuli such as temperature or ultravioletirradiation. One such ionomer, poly(ethylene-co-methacrylic acid)[EMAA], also known under the trade name Surlyn®, undergoes puncturereversal (self-healing) following high-velocity ballistic penetration.The heat generated from the damage event triggers self-healing in thismaterial. Although EMAA polymers possess excellent puncture healingproperties, their low tensile modulus (308 MPa) limits their use as anengineering polymer in structural aerospace applications.

Also, a self-healing composite laminate system that possesses aerospacequality consolidation with fiber volume fraction (FVF) of up to 57percent and void volume fraction of less than two percent does notcurrently exist. Most self-healing composite laminates that have beenreported possess 20-30 percent fiber volume, which is well belowaerospace industrial standards for fiber-reinforced composites (FRC).

A need exists for an inherently self-healing composite laminate matrixthat does not rely on foreign inclusions for self-repair and which hasmechanical properties with potential for aerospace applications. A needfurther exists for an appropriate process for making such a matrix, foran appropriate process for making CFRCs from such a matrix, and forrepairing such CFRCs.

SUMMARY OF THE INVENTION

A composite comprising a combination of a polybutadiene graft copolymermatrix and a carbon-fiber reinforcement is provided. Additionally, amethod of fabricating a composite comprising the steps of manufacturinga pre-impregnated unidirectional carbon fiber preform by wetting aplurality of carbon fibers with a solution, the solution comprising aself-healing polymer and a solvent, and curing the preform, is provided.Moreover, a method of repairing a damage composite of the invention isprovided. Other features and advantages will become apparent upon areading of the attached specification, in combination with a study ofthe drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The organization and manner of the structure and operation of theinvention, together with further objects and advantages thereof, maybest be understood by reference to the following description, taken inconnection with the accompanying drawings, wherein like referencenumerals identify like elements in which:

FIG. 1 is a diagram of the chemical structure of the polybutadiene graftcopolymer of the preferred embodiment of the invention.

FIG. 2 is a graph of a differential scanning calorimeter (DSC) scan ofpristine polybutadiene graft copolymer (PBg) amorphous thermoplastic.

FIG. 3 is a graph of dynamic temperature taken using athermo-gravimetric analyzer (TGA) of pristine PBg amorphousthermoplastic.

FIG. 4 is a graph of mass evolution in dynamic temperature TGA scan ofstructural reinforcing IM7 carbon fiber (IM7)/PBg prepreg containinganhydrous N-Methylpyrrolidone (NMP) solvent.

FIG. 5 is a graph of isothermal mass evolution of pristine PBg andIM7/PBg prepreg.

FIG. 6 is a dynamic temperature viscosity profile of the pristine PBgthermoplastic.

FIG. 7 is a graph of the isothermal temperature and dynamic viscosity ofPBg-NMP solution.

FIG. 8 is a photomicrograph at 50× of IM7 5-harness satin biaxial/PBgmatrix composite by resin film infusion (RFI).

FIG. 9A is an optical micrograph at 100× of IM7/PBg.

FIG. 9B is an optical micrograph at 100× of IM7/977-3.

FIG. 10A is a through-transmission c-scan of IM7/PBg panel post-impact.

FIG. 10B is a through-transmission c-scan of the panel of FIG. 10A,post-healing cycle.

FIG. 11 is a graph of IM7/PBg testing results of pristine couponcompression, barely visible impact damage (BVID) coupon compressionafter impact (CAI) and BVID-Healed coupon compression prepared from twobatches of experimental prepreg (TM340 and TM341).

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

While the invention may be susceptible to embodiment in different forms,there is shown in the drawings, and herein will be described in detail,specific embodiments with the understanding that the present disclosureis to be considered an exemplification of the principles of theinvention, and is not intended to limit the invention to that asillustrated and described herein. Therefore, unless otherwise noted,features disclosed herein may be combined together to form additionalcombinations that were not otherwise shown for purposes of brevity.

In one embodiment, a polybutadiene graft copolymer (PBg) was selected asa matrix in carbon-fiber-reinforced composites due to its highermechanical and thermal properties as compared to the other self-healingthermoplastics which have been studied. According to material suppliers,PBg has a glass transition temperature (T_(g))=80 degrees Celsius (°C.), room temperature (RT) tensile strength of 37 MPa, RT tensilemodulus of 2.47 GPa, and a 7.5 percent elongation at break. The tensilemodulus of the neat polymer is about 10 percent lower than the 2.76 GParequired of matrix polymers typically used in aerospace primarystructural applications. In one embodiment, the matrix is apolybutadiene graft copolymer matrix, such as a polybutadiene graftcopolymer comprising poly(butadiene)-graft-poly(methylacrylate-co-acrylonitrile). In an embodiment, the self-healing polymercomprises poly(butadiene)-graft-poly(methyl acrylate-co-acrylonitrile).In some embodiments, the matrix is a soluble matrix that allows forprocessing of the self-healing thermoplastic composite. In furtherembodiments, the solvent used to process the matrix isN-methylpyrrolidone.

Structures using self-healing polymer matrices such as, for example, PBgprovide the following advantages: (1) increased damage tolerancecompared to thermosetting polymer matrices which incur a greater extentof impact damage as compared to PBg composites, based upon C-scan data;(2) Self-repairing polymer matrices provide a route for recovery of alarge proportion of the pristine mechanical properties, thus extendingthe life of the structure; (3) Since self-healing is an intrinsicproperty of the matrix material, this matrix can be treated a directsubstitute to conventional thermosetting matrices that do not possessself-healing characteristics; (4) The intrinsic healing of PBg does notrequire the introduction of microcapsules needed by other healingpolymer matrices described in the prior art section; it therefore hasthe advantage of not needing microcapsules which can act as defectinitiators in the composite; (5) As long as there is no loss of matrixmaterial incurred in the damage event, PBg can self-heal repeatedly,compared to the microcapsule approach, where healing is limited by theamount of monomer present at the site where damage occurs. In someembodiments, the self-healing polymer is able to heal crack(s) and/orpuncture(s) in the structure.

A composite fabrication process cycle was developed from compositeprecursor materials to fabricate composite laminates for aerospacestructures. The precursor material is a pre-impregnated unidirectionalcarbon-fiber preform, also known as a prepreg. In the prepreggingprocess, the high-strength, structural reinforcing IM7 carbon fiber iswetted by a solution containing the self-healing PBg polymer dissolvedin N-Methylpyrrolidone (NMP). Precursor materials in the form ofunidirectional prepreg are widely used in the aerospace industry tofabricate and manufacture aerospace composite structures for bothcommercial and military aviation and space launch vehicle systems. Theexperimental prepreg containing the puncture-self-healing polymer PBgwas characterized and a cure cycle developed to be used in fabricationof composite laminate test coupons from this novel prepreg material.

A self-healing thermoplastic PBg, poly(butadiene)-graft-poly(methylacrylate-co-acrylonitrile), a polybutadiene graft copolymer, wasobtained in pellets from Sigma-Aldrich. The chemical structure of thisPBg is shown in FIG. 1. Initially CFRC processing attempts with the PBgthermoplastic were conducted using IM7-6K 5-harness satin woven fabric(GP sizing, 280 gsm) from Textile Products, Inc., Anaheim, Calif., USA.Experimental batches of PBg prepreg were developed using IM7-12K unsizedfiber tow from Hexcel and anhydrous N-Methylpyrrolidone (“NMP”) solventsupplied by BASF Chemical Co, Florham Park, N.J. Quasi-isotropic panelswere fabricated for the purpose of consolidation quality comparisonusing Cycom® IM7/977-3 prepreg supplied by Cytec Engineered Materials,Woodland Park, N.J.

As a first step in determining the CFRC processing temperature window,the glass transition (T_(g)) of the PBg polymer pellets was verified byconducting a dynamic temperature scan in nitrogen from 25° C. to 300° C.at 5° C./min in a Netzsch 204-F1 Phoenix® differential scanningcalorimeter (DSC). In addition to the dynamic scan in DSC, dynamictemperature scans in nitrogen from 25° C. to 300° C. at 5° C./min wereconducted using a Netzsch TG-209-F1 Libra® thermo-gravimetric analyzer(TGA) to determine the decomposition temperature of the pristine PBgpolymer.

Residual solvent content in IM7/PBg prepreg was obtained from end-rollspecimens of the prepreg batch, designated tape machine run # (TM-341),using a dynamic temperature scan from a thermo-gravimetric analyzer(TGA) under a nitrogen purge from 25° C. to 300° C. at five ° C./min.The TGA was a Netszch TG-209-F1 Libra® TGA. This data was also used todetermine the temperature which would be required to remove thevolatiles (NMP) from the IM7/PBg prepreg during CFRC processing. Usingthese results to initially select an isothermal dwell temperature in theCFRC processing prior to the compaction step, the time duration of thisproposed dwell was determined by performing an isothermal scan at 150°C. and 225° C. in TGA of the IM7/PBg prepreg. Using a specimen from theTM341 roll of IM7/PBg prepreg, the mass evolution of the material duringthe proposed CFRC process cycle was determined in the Netzsch TGA byheating to 150° C. at 5° C./min and holding for one hour under nitrogenpurge and then heating from 150° C. to the mold compaction temperatureof 225° C. at five ° C./min and holding for two hours.

The PBg polymer was molded into neat resin disks 2.54 cm in diameter by1.5 millimeters (mm) in thickness for rheological characterization in aRheometrics ARESV® parallel plate rheometer. All of the rheology resultspresented in this study were collected using a cyclic strain of twopercent. A dynamic temperature scan in nitrogen was conducted from 25°C. to 285° C. at 5° C./min. Based on the results of the TGA thermogram,an isothermal temperature scan at 150° C. and 225° C. was performed onthe solution of NMP containing 31 percent solids PBg to determine thechange in the dynamic viscosity as the matrix material devolatilizesduring the proposed processing cycle.

Pellets of the PBg polymer were dissolved in anhydrous NMP bycontinuously stirring for 48 hours at 25° C. under nitrogen purge. Theresulting uniform mixture contained 31 percent solids in NMP by weight.The Brookfield viscosity of the resulting solution at 25° C. wasdetermined to be 21.12 Pa*sec (211.20 poise). This viscosity wasacceptable for the solution prepregging process. Two experimentalbatches of PBg prepreg were fabricated with prepregging equipment. The22 to 25 centimeters (cm) wide prepregs were fabricated using 90 unsizedIMT-12K tows by introducing the PBg-NMP polymer in solution to theunsized IM7 fiber via the dip tank in the prepregging process. Using anestablished procedure of weighing, oven drying, and reweighing samplesof the prepreg, the resulting fiber areal weight (FAW), PBg resin andNMP solvent content of these two experimental batches were determined.

The processing cycle determined following the above tests was then usedto fabricate three [45/0/−45/90]_(4S) IM7/PBg panels includinggeometries of 7.6 cm×7.6 cm, 15.2 cm×15.2 cm, and 30.5 cm×30.5 cm.Material from the first prepreg batch was processed in stainless steelclosed molds using a TMP® three-ton vacuum press with a layer ofbreather and release cloth separating the stack of prepreg from thestainless steel mold base and plunger. A 15.2 cm×15.2 cm[45/0/−45/90]_(4S) panel was fabricated in the same mold and vacuumpress using Cytec IM7/977-3 toughened epoxy prepreg and the Cytecrecommended processing cycle, C-49. Both the IM7/PBg and the IM7/977-315.2 cm×15.2 cm panels were cross-sectioned at the center using a wetsaw and then potted and polished for optical microscopy in a ReichertMEF4 M microscope. Following ASTM D3171, FVF/VVF analysis by aciddigestion were conducted for this IM7/PBg panel and three subsequentlyprocessed 15.2 cm×15.2 cm IM7 PBg panels. Based on these results, sixadditional [45/0/−45/90]_(4S) 15.2 cm×15.2 cm IM7/PBg panels werefabricated from prepreg batch (TM-340) and six from prepreg batch(TM-341) in the vacuum press for the purpose of determining thecompression after impact (CAI) strength of these composite materialsafter low-velocity impact damage.

Nine IM7/PBg panels were prepared as test coupons and subjected tolow-velocity impact according to ASTM D7136. A spherical tup was used toimpact each 15.2 cm×10.1 cm coupon at the center. The average couponthickness of the six panels fabricated from prepreg batch TM340 was 5.40mm, and an impact energy of 36.08 J was used to damage these coupons.The six panels fabricated from prepreg batch TM-341 had an averagethickness of 4.62 mm. Four of these six panels were damaged using animpact energy of 31.09 J. Non-destructive evaluation (NDE) bythrough-transmission, time-of-flight c-scan of these impacted panels wasconducted using a Sonotek® c-scan with a 10 MHz transducer. After c-scanof all of the damaged coupons, one of the coupons from the panelsfabricated using the TM-340 prepreg batch and three of those fabricatedusing the TM-341 batch were randomly selected and subjected to anelevated temperature/pressure healing cycle in the vacuum press usingthe following cycle:

25° C. to 225° C. at five ° C./min under full vacuum;

hold at 225° C. for 30 minutes under full vacuum and 1.7 MPa pressure;and

cool down the vacuum press to 25° C. at five ° C./min under full vacuum.In some embodiments, the vacuum press is cooled such that the samplecools slowly while still under full vacuum.

Both pristine and damaged IM7/PBg quasi-isotropic laminates were testedaccording to ASTM D7137 using a CAI test fixture in an MTS 250KN LoadFrame. In addition, the pristine compression strength of the IM7/PBg wasdetermined by mounting these pristine 15.2 cm×10.1 cm coupons in the CAIfixture and loading them in axial compression.

The T_(g) of the polymer was determined at the inflection in the heat vstemperature curve shown in FIG. 2. This measured value of 75° C. is veryclose to the vendor-specified value of 80° C. A significant reduction inthe modulus of the polymer is associated with this transition. Forexample, the tensile modulus of the PBg polymer at 25° C. of 2.5 GPa isreduced to 2.2 MPa at 100° C. as reported by the material supplier.

The results of the dynamic temperature scan in TGA shown in FIG. 3indicates a pristine PBg sample mass loss of 2 percent at 300° C. as thedecomposition temperature and indicates that the polymer can beprocessed at temperatures up to about 300° C. without significantdegradation.

Residual solvent trapped in the prepreg will result in composite partswith high void content. The thermogravimetric scan shown in FIG. 4indicates that about 7 percent solvent evolved from the prepreg between100° C. and 200° C.

Using these results, an isothermal dwell temperature of 150° C. in theCFRC processing cycle was initially selected to devolatilize the IM7/PBgprepreg prior to the compaction step at 225° C. The temperature of 150°C. also coincides with the reduced viscosity determined by rheologicalanalysis. The time required in the proposed devolatilization dwell wasinvestigated by isothermal TGA of the IM7/PBg prepreg. Shown in FIG. 5are the mass evolution of both the pristine PBg polymer from pellet andthe IM7/PBg prepreg during the proposed one-hour isothermal drying stepat 150° C., as well as the two-hour compaction step at 225° C. Theisothermal TGA scans in FIG. 5 indicate a mass loss of one percent inthe pristine PBg polymer after one-hour hold at 150° C., while theIM7/PBg prepreg lost up to 5 percent mass. This result indicates a netdevolatilization of NMP solvent of about 4 percent by weight leaving apossible residual six percent by weight (w) solvent in the prepreg,based on the total amount of solvent left in the prepreg after theprepregging process, going into the temperature ramp to the compactionand consolidation step at 225° C. This level of solvent content couldresult in void entrapment during the compaction phase. The highviscosity of the PBg polymer might prevent the full removal of NMP,regardless of the devolatilization step duration.

Having determined the T_(g) and the decomposition temperature of the PBgpolymer, the CFRC processing temperature window exists between about 75°C. and about 300° C. Thermoplastic polymers are typically difficult toprocess as matrix in CFRC because the long molecular chains of thesepolymers make the bulk material highly viscous. Knowing the thermalprocessing window, the dynamic viscosity versus temperature of the PBgpolymer was determined via parallel plate rheology. Thedynamic-temperature rheology scan in FIG. 6 indicates that the PBgpolymer exhibits two separate events where the heat introduced to thematerial results in significant reduction of the dynamic viscosity, η*.Following the stick-slip phenomenon in the first 20 minutes of the test,the first event occurs near 150° C. where the dynamic viscositydecreased from a maximum of 5,000 Pa*sec (50,000 poise) to 1,500 Pa*sec(15,000 poise). The second event occurs at about 260° C. where the PBgpolymer reaches its minimum dynamic viscosity of 360 Pa*sec (3,600poise). However, the rapid increase in the viscosity at temperaturesabove about 275° C., combined with the two percent mass loss observed inTGA indicates that the polymer is beginning to degrade at this elevatedtemperature. Therefore a maximum molding temperature of 225° C. withviscosity of 1,300 Pa*sec (13,000 poise) was selected to process the PBgas matrix in CFRC composites.

After determining the mass evolution in TGA of the IM7/PBg prepregduring the proposed CFRC processing devolatilization step at 150° C. andcompaction step at 225° C., the effect of these processing dwells on thedynamic viscosity of the PBg polymer was investigated in parallel platerheology. The results of this isothermal temperature scan performedusing the PBg-NMP prepregging solution are shown in FIG. 7. During theisothermal hold at 150° C. the PBg containing residual NMP exhibited arelatively stable viscosity of 1,500 Pa*sec (15,000) poise. Asignificant decrease in dynamic viscosity was observed during the five °C./min temperature ramp to 225° C. During the 60-minute hold at 225° C.,the viscosity of the material was less stable, increasing from 6,700poise at the beginning to 12,300 poise by the end of the isothermalhold. The change in viscosity may be due to devolatilization of the NMPsolvent, which has a boiling point of 200° C. at atmospheric pressure orthe increase may be due to some initial degradation of the PBg molecule.

The resin film infusion (RFI) process is preferable to alternativemethods that require an intermediate step of pre-impregnating carbonfiber tows with resin. However, the photomicrograph in FIG. 8 shows thatthis method was not suitable for PBg.

It is noted that the PBg resin was able to flow around the IMT-6K fibertows in the biaxial fabric, but does not penetrate the tows. In fact thetows which are 90-degrees to the plane are clearly surrounded by aregion of voids. Based on these results, the effort to fabricate CFRCwith PBg thermoplastic matrix focused on the development of a processingmethod with an intermediate prepreg material.

Two experimental batches of PBg prepreg were fabricated usingprepregging equipment. The fiber areal weight (FAW), PBg resin and NMPsolvent content of these two experimental batches are displayed in Table1.

TABLE 1 Characteristics of NASA LaRC IM7/PBg unidirectional prepreg.Resin Resin Solvent Run Viscosity, FAW, Content, Content, Width, Length,Number Poise g/m² wt % Dry wt % Wet cm m TM-340 210 159 41-43 15-20 2566 TM-341 211 146-150 34-35 10-11 22 59

Based on the results obtained in the thermal and rheological analysis ofthe IM7/PBg experimental prepreg, the processing cycle in the vacuumpress for the IM7/PBg composite was selected to be:

-   -   1. 25° C. to 150° C. at two ° C./min under full vacuum, hold at        150° C. under full vacuum for 60 minutes;    -   2. 150° C. to 225° C. at two ° C./min under full vacuum, hold at        225° C. for 60 minutes under full vacuum and 1.7 MPa compaction        pressure during entire temperature hold;    -   3. Cool down to 25° C. at two ° C./min under full vacuum.

Using these conditions, three [45/0/-45/90]_(4S) panels measuring 7.6cm×7.6 cm, 15.2 cm×15.2 cm, and 30.5 cm×30.5 cm were fabricated. Uponvisual inspection, all three of these panels exhibited higherconsolidation quality than the previous RFI panels. Both the IM7/PBg andthe IM7/977-3 15.2 cm×15.2 cm panels were cross-sectioned at the centerusing a wet saw and then potted and polished for photo microscopy. Theresulting images are shown side-by-side for comparison in FIG. 9.

The micrographs in FIG. 9 of the center plies of the 32-ply quasi panelsindicate that the IM7/977-3 composite was very well consolidated andessentially void free, or consistent with VVF less than two percent. Theplies had uniform thickness of 0.127 mm and the fibers were uniformlydispersed with the epoxy matrix in each of the plies. The micrograph ofthe IM7/PBg composite at the same stack location show that the plieswere fairly uniform with an average thickness of about 0.183 mm, butthere were some resin fiber discontinuities, or resin-rich regions.There were several small voids evident in this small sampling of theoverall composite. Photomicrographs of additional IM7/PBg compositesalso contained voids, especially near panel edges. These were likelyformed by the NMP volatiles trapped in the highly viscous polymer. Voidcontent analysis by acid digestion, using a polymer density of 1.31g/cc, of three subsequent 32 ply quasi-isotropic 15.2 cm×15.2 cm IM7/PBgpanels revealed an average FVF>60 percent, and average VVF<two percent.Based on these results, additional IM7/PBg panels from both experimentalbatches of prepreg were fabricated in the vacuum press for the purposeof determining the compression after impact (CAI) strength of thesenovel composite materials.

Nine IM7/PBg panels were subjected to low-velocity impact resulting inan average damage dent depth of 1.9 mm (0.075 in). The damage regions ofall the impacted coupons were analyzed by c-scan. A representative imageof the damage incurred in the IM7/PBg panels is shown in FIG. 10A. NDEof these IM7/PBg coupons indicated an average planar delamination areaof 15.3 cm². The damage area and dent depth are consistent with barelyvisible impact damage (BVID).

Four of the BVID IM7/PBg panels were subjected to an elevatedtemperature/pressure healing cycle described above and then tested tofailure in compression to determine the influence of the cycle on theIM7/PBg composite CAI failure strength. The time-of-flight c-scan imageof one of these IM7/PBg panels before and after, healing is shown inFIGS. 10A and 10B. As a result of the elevated temperature/pressurehealing cycle, no apparent damage was evident in the c-scan. After thehealing cycle, the 1.9 mm deep dent on the surface of the panel was nolonger visible.

The results of the axial compression of the two pristine IM7/PBgcoupons, the five BVID coupons and the four BVID coupons subjected to anelevated temperature/pressure healing cycle are shown in FIG. 11.

All of the coupons failed due to fiber micro-buckling. In the five BVIDcoupons, this failure initiated at the site of the impact damage andpropagated across the width of the coupons. The four IM7/PBG couponscontaining BVID which were subjected to an elevated temperature/pressurehealing cycle also failed due to fiber microbuckling initiating at theoriginal impact site and propagating across the 10.2 cm width of thecoupons. The pristine compressive strength resulting from this limitedsample of coupons of quasi-isotropic laminates was about 52 percent ofthe compressive strength of the 675 MPa reported for a typical toughenedepoxy 32-ply quasi-isotropic CFRC intended for aerospace structure.

The CAI strength of the BVID coupons appears to be independent of theprepreg batch. This is expected since the failure of these coupons isdominated by the impact damage and in this study the impact energy usedfor BVID was varied to account for the panel thickness variationresulting from prepreg batch inconsistency. The inconsistency of theprepreg batches was not of significant concern at this initial phase inthe study given that all of the IM7/PBg prepreg were consideredexperimental. The differences in the batch to batch quality is noted inTable 1 and demonstrates that there is considerable room for improvementin both the quality of the prepreg and the resulting mechanicalperformance of CFRC panels fabricated from the intermediate material. Inaddition to the inconsistencies in the experimental prepreg, thefiber/matrix interface is not optimized with fiber sizing used tooptimize the interfacial adhesion in toughened epoxy CFRCs.

Regardless of prepreg batch, there was a significant improvement in thefailure strength of the BVID coupons subjected to an elevatedtemperature/pressure healing cycle vs the BVID panels which were nothealed. The TM340 batch of damaged IM7/PBg panels exhibited a 64 percentretention of compressive strength. The retention of compressive strengthof coupons fabricated from this same batch of prepreg, which weresubjected to BVID and then the elevated temperature/pressure healingcycle was found to be 80 percent. However, the large error associatedwith the BVID-Heal coupons from the second prepreg batch (B2) indicatesthat this notable improvement in compressive strength may be just asdependent on the quality of the CFRC laminate as it is on the healingcapability of the matrix. The non-autonomic healing cycle utilized inthis initial study amounts to reprocessing of the amorphousthermoplastic PBg matrix in the IM7/PBg composite.

The composite of the present invention will be useful for structures inapplications such as, but not limited to, structural components that canbe incorporated into vehicles such as aircraft, rotorcraft, andspacecraft. Particular applications that can include these structuralcomponents are by way of example and not limitation, combat aircraft,large military transport and bomber aircraft, commercial transport,small transport, general aviation, military rotorcraft, commercial andgeneral aviation rotorcraft, spacecraft, and missiles.

While preferred embodiments of the present invention are shown anddescribed, it is envisioned that those skilled in the art may devisevarious modifications of the present invention without departing fromthe spirit and scope of the appended claims.

What is claimed is:
 1. A method of fabricating a composite comprisingthe steps of: dissolving a polybutadiene graft copolymer with a solventto form a solution; wetting a plurality of tows of a carbon fiber or afabric of the carbon fiber with the solution; fabricating prepregs fromthe wetted tows or fabric in a prepreg machine; and laying up fabricatedprepregs and process in a vacuum press to produce the composite, whereinthe composite exhibits an average fiber volume fraction (FVF) of greaterthan 60%.
 2. The method of claim 1, wherein the fabricating stepcomprises: increasing the temperature in the vacuum press from about 25°C. to about 150° C. at about 2° C. per minute under full vacuum andholding at about 150° C. for about 60minutes under full vacuum;increasing the temperature in the vacuum press to about 225° C. at about2 ° C. per minute under full vacuum and holding at about 225° C. forabout 60 minutes under full vacuum and about 1.7MPa compaction pressure;and cooling the vacuum press to about 25° C. at about 2 ° C. per minuteunder full vacuum.
 3. The method of claim 1, wherein the polybutadienegraft copolymer comprises poly(butadiene)-graft-poly(methylacrylate-co-acrylonitrile).
 4. The method of claim 1, wherein thecomposite exhibits an average void volume fraction (VVF) of less than2%.